Today we have another analysis of Android device security against scenarios where the owner loses control of the phone, by the same researchers who wrote Security Analysis of Android Factory Resets. Here, instead of looking at what happens when the owner deliberately erases a phone in order to sell it, they study what happens when the phone is stolen and the owner tries to wipe or disable it remotely. A remote disable mechanism is commonly included in anti-virus programs for Android, and this study looks at ten such mechanisms.

As with factory reset, the core finding boils down to none of them work. The root causes are slightly different, though. The core of Android, naturally, is at pains to prevent normal applications from doing something as destructive as initiating a memory wipe, rendering the phone unresponsive to input, or changing passwords. To be properly effective the anti-theft program needs administrative privileges, which can be revoked at any time, so there’s an inherent race between the owner activating the anti-theft program and the thief disabling it. To make matters worse, UX bugs make it easy for these programs to appear to be installed correctly but not have the privileges they need; implementation bugs (possibly caused by unclear Android documentation) may leave loopholes even when the program was installed correctly; and several device vendors added backdoor capabilities (probably for in-store troubleshooting) that allow a thief to bypass the entire thing—for those familiar with Android development, we’re talking if the phone is turned off and then plugged into a computer it boots into recovery mode and activates ADB.

There is a curious omission in this paper: since 2013, Google itself has provided a remote lock/wipe feature as part of the Google Apps bundle that’s installed on most Android devices [1]. Since the Apps bundle is, nowadays, Google’s way of getting around vendors who can’t be bothered to patch the base operating system in a timely fashion, it has all the privileges it needs to do this job correctly, and the feature should be available regardless of Android base version. The UX is reasonable, and one would presume that the developers are at least somewhat less likely to make mistakes in the implementation. This paper, however, doesn’t mention that at all, despite (correctly) pointing out that this is a difficult thing for a third-party application to get right and the device vendors should step up.

Practical advice for phone-owners continues to be: encrypt the phone on first boot, before giving it any private information, and use a nontrivial unlock code. Practical advice for antivirus vendors, I think, is you’re not testing your software adversarially enough. The implementation bugs, in particular, suggest that the vendors’ test strategy confirmed that remote lock/wipe works, when set up correctly, but did not put enough effort into thinking of ways to defeat the lock.

Unlike the last two papers, today’s isn’t theoretical at all. It’s a straightforward experimental study of whether or not any data can be recovered from an Android phone that’s undergone a factory reset. Factory reset is supposed to render it safe to sell a phone on the secondhand market, which means any personal data belonging to the previous owner should be erased at least thoroughly enough that the new owner cannot get at it without disassembling the phone. (This is NIST logical media sanitization, for those who are familiar with those rules—digital sanitization, where the new owner cannot extract any data short of taking an electron microscope to the memory chip, would of course be better, but in the situations where the difference matters, merely having a cell phone may mean you have already failed.)

The paper’s core finding is also straightforward: due to a combination of UI design errors, major oversights in the implementation of the factory-reset feature, driver bugs, and the market’s insistence that flash memory chips have to pretend to be traditional spinning rust hard disks even though they don’t work anything like that, none of the phones in the study erased everything that needed to get erased. The details are confusingly presented, but they’re not that important anyway. It ends with a concrete set of recommendations for future improvements, which is nice to see.

There are a few caveats. The study only covers Android 2.3 through 4.3; it is unclear to me whether the situation is improved in newer versions of Android. They don’t discuss device encryption at all—this is at least available in all versions studied, and should completely mitigate the problem provided that a factory reset successfully wipes out the old encryption keys, and that there’s no way for unencrypted data to survive the encryption process. (Unfortunately, the normal setup flow for a new phone, at least in the versions I’ve used, asks for a bunch of sensitive info before offering to encrypt the phone. You can bypass this but it’s not obvious how.) It would be great if someone tested whether encryption is effective in practice.

Beyond the obvious, this is also an example of Android’s notorious update problem. Many of the cell carriers cannot be bothered to push security updates—or any updates—to phones once they have been sold. [1] They are also notorious for making clumsy modifications to the software that harm security, usability, or both. [2] In this case, this means driver bugs that prevent the core OS from erasing everything it meant to erase, and failure to deploy patches that made the secure erase mechanism work better. Nobody really has a good solution to that. I personally am inclined to say that neither the telcos nor Google should be in the business of selling phones, and conversely, the consumer electronics companies who are in that business should not have the opportunity to make any modifications to the operating system. Whatever the hardware is, it’s got to work with a stock set of drivers, and the updates have to come direct from Google. That would at least simplify the problem.

(Arguably Google shouldn’t be in the OS business either, but that’s a more general antitrust headache. One thing at a time.)